It is known that organic optoelectronic devices such as OLEDs must be encapsulated in order to protect their sensitive components against gas species in the atmosphere (mainly oxygen and water vapor). This is because, if this protection is not provided in a suitable way, there is a risk of further degradation of the device, manifested mainly in the appearance of non-emissive black spots in the case of OLEDs, which are in fact the result of the penetration of water vapor into the diode, which degrades the interface between the cathode (or anode) and the organic film or films.
This encapsulation may typically be provided by using a glass lid bonded onto the organic device by means of a special adhesive which has, notably, low water permeability. As a general rule, a solid moisture absorber or “getter” is added between the substrate and the lid to extend the lifetime of the device.
For some applications, and also in order to reduce costs, thin layers with a barrier action have been developed, for the purpose of protecting the underlying device against damage by moisture, in a similar way to the lid and getter assembly. As a general rule, these barrier layers are oxides, nitrides or oxynitrides, or, in some cases, thin metallic layers, unless the electroluminescent unit is what is known as “top emission” unit, emitting from the top of the structure, in which case the barrier layers must be transparent.
These thin encapsulation layers typically have a total thickness of less than 1 μm, and are deposited by standard vacuum deposition methods such as chemical vapor phase deposition (CVD), which may be plasma assisted (PECVD), atomic layer deposition (ALD, sometimes called AL-CVD), or physical vapor phase deposition (PVD) including evaporation and spraying. It is difficult to envisage the deposition of these thin encapsulation layers by means of other, less time-intensive types of deposition, such as liquid phase deposition, because this type of deposition requires the use of polymer solutions containing solvents which may dissolve the layers of the underlying electroluminescent unit.
In the specific case of hybrid OLED units, in other words those in which the stack of organic films interleaved between the electrodes includes among its innermost active films one or more films deposited by a liquid route (typically by what is known as “spin coating”), so that the control of deposition is simpler than in the case of printing techniques, the problem of the precise location of these films arises, this problem being of major importance for the production of large numbers of micro-screens on a silicon slice. This is because these films, such as electron transport layer (ETL), hole injection layer (HIL) and hole transport layer (HTL) films, are deposited by a liquid route over the whole substrate (that is to say, over the whole surface of the silicon slice) and therefore cannot be precisely located (that is to say, confined to the OLED unit) unless photolithography is performed subsequently, which is unfortunately less feasible because of the fragility of the components of OLED units when subjected to etching. Now, it is known that these films deposited by a liquid route, if they are not subsequently confined to the active zone of the corresponding micro-screen, will adversely affect the quality of encapsulation of the OLED unit because they form a lateral channel for the ingress of ambient atmospheric water vapor into the OLED unit by permeation, and because the encapsulation deposited onto these films does not adhere satisfactorily to them.
The document WO-A1-2009/101299 in the name of the present applicant discloses the use, in a thin-layer encapsulation of an organic optoelectronic device, of a continuous moisture-reactive layer based on an organometallic complex such as tris(8-hydroxyquinolinato)aluminum (III) (Alq3), and a barrier layer surmounting it and that may consist of an oxide chosen from among those conforming to the formulae Al2O3, SiO2, SixNy and SiOxNy.
The document US-A-2006/0061272 shows, in its FIG. 3, an optoelectronic device comprising a barrier layer, based on silicon monoxide for example, covering a buffer layer, based on Alq3 for example. The buffer layer is placed on top of an external electrode and a stack of light-emitting organic films, and the barrier layer is surmounted by an encapsulation with two inorganic layers and is spaced apart laterally from the stack of films in this figure, by the buffer layer on the right and by the external electrode and this buffer layer on the left.